Location estimation is essential to many smartphone applications targeting context-aware services in indoor environments [1]}, [2]}.
Most of these applications exploit wireless signals to passively estimate the user's location in the background while having the context-aware services run in the foreground.
There are two common approaches to wireless-based location estimation: direct approach and indirect approach. The direct approach is a single-step estimation that estimates the location by directly processing the wireless signals; whereas the indirect approach involves an intermediate step in computing the distance, time of arrival (ToA), or angle of arrival (AoA) prior to location estimation.
The computed distance, ToA, and AoA are then used as input parameters by trilateration or triangulation algorithms to estimate the target location [3]}, [4]}, [5]}, [6]}, [7]}.
Recently, location estimation based on the direct approach has attracted a lot of interest because it can estimate the location directly without going through the intermediate step.
The direct approach is achieved by exploiting wireless signals from densely deployed transmitting and receiving sources. For example, [8]}, [9]} use a large-scale antenna array and [10]} uses the received signal from multiple base stations to achieve direct localization.
